staging: ccree: NULLify backup_info when unused

backup_info field is only allocated for decrypt code path.
The field was not nullified when not used causing a kfree
in an error handling path to attempt to free random
addresses as uncovered in stress testing.

Fixes: 737aed94 ("staging: ccree: save ciphertext for CTS IV")
Cc: stable@vger.kernel.org
Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>